In conjunction with the control of a wheel brake, it is important to know when and/or whether the brake linings of the brake disc or brake drum lift up or have lifted up. This information is the basis for setting a defined air clearance on the wheel brake and contributes to the safety and availability of the wheel brake. Using a disc brake as an example, for correct ventilation of the wheel brake when the brake pedal is not activated, the brake piston as well as the floating caliper must lift a few tenths of a millimeter from the brake disc. For this purpose, the piston sealing rings, which ensure the return of the brake piston, as well as the guides of the floating caliper must be completely functional. However, the floating caliper tends to block as age and soiling increase as well as due to asymmetrical distribution of mass that may be present in newer brake systems such as, for example, a wheel brake with electromotive braking. The blockage by the floating caliper exerts a residual force on the brake disc via the lining on the outer side of the brake that is supported by the guide pins of the floating caliper. A constant one-sided heating of the brake disc and the brake lining causes, in extreme cases, the brake disc to warp due to the continuous stress, and the fading reserves can be quickly exhausted in a subsequent braking due to the high initial temperature. Furthermore, wear of the brake lining increases disproportionately, so that new linings must be prematurely installed. This residual force or residual braking torque cannot be decreased using control. Similar problems occur in other types of brakes as well, such that there is a need for having the above-mentioned information for these brake types as well. On the other hand, the sensors used for measuring the braking torque display drifting phenomena, so that it is not possible to determine using the sensor signal alone whether and when the brake linings lift or have lifted from the brake disc or brake drum.
A method is described in German Patent Application No. 198 26 053, with the aid of which the offset of a braking torque sensor can be detected. In addition to the braking torque, a quantity for the value (path) covered by the brake lining and the tendency of the braking torque to change along the path are calculated. If the torque no longer changes substantially along the path, the lining is assumed to have lifted. The air clearance is adjusted based on this zero point. Whether and when actual ventilation of the wheel brake has occurred is not determined.
By determining a residual braking torque, an increase in the driving safety of the vehicle is achieved because damage, in particular deformation, of the braking disc caused by one-sided heating, is effectively prevented and fading reserves are preserved. The driver is informed in a timely manner of the condition of the wheel brake and of increased wear on a brake lining and the brake disc, in particular in the case of floating caliper disc brakes.
It is especially advantageous that not only detection of a residual braking torque and recognition of a blocking wheel brake are performed, but also the offset of the sensor signal is detected in order to compensate for a drifting sensor offset.
Thus, the driving safety and availability of the wheel brake as well as the accuracy of the sensor signal are improved overall.